1. Field of the Invention
The present invention relates in general to mixed signal processing and, in particular, to delta-sigma modulation.
2. Background of the Invention
Delta-sigma modulators are particularly useful in digital to analog and analog to digital converters (DACs and ADCs), as well as codecs and similar mixed signal applications. Generally, delta-sigma modulators, which can operate on either an analog or digital input, generate a quantized output that tracks the average of the input signal. Using oversampling, the delta-sigma modulator spreads the quantization noise power across an oversampling frequency band, which is typically much greater than the input signal bandwidth. Additionally, delta sigma modulators perform noise shaping by acting as a highpass filter to the noise such that most of the quantization noise power is shifted out of the signal band of interest.
The typical delta sigma modulator sums the input signal with negative feedback, performs a linear filtration operation and then a one-bit or multiple-bit quantization. In a first order modulator, the linear filter includes a single integrator stage while the filter in a higher order modulator has a cascade of a corresponding number of integrator stages. Higher order modulators have the advantage of improved noise shaping capability over lower order modulators, although stability becomes a more critical consideration as the order is increased.
During device power-up and power-down, discontinuities can appear in the delta-sigma modulator output. In audio applications, these discontinuities manifest themselves as audible clicks and pops to the listener. Hence, “pop-guard” techniques are typically employed in low-end audio delta-sigma DACs, such as those used to drive headphones and inexpensive speakers.
One common pop-guard technique ramps-up and ramps-down the modulator input during power supply transitions to minimize output discontinuities. For a single-ended modulator configuration, the “quiet” signal level is typically in the middle of the power supply voltage range. Thus, for this configuration, the target is to ramp the output to and from this mid-range voltage level by corresponding ramping of the input voltage.
As further discussed below, to maintain modulator stability, the peak-to-peak output relative to the power supply rails is limited, which consequently limits the ability of the modulator to smoothly ramp the output from the power supply rails. This limitation can itself cause discontinuities in the output and is therefore a significant problem, which must be addressed.